1. Field of the Invention
This invention relates to photovoltaic materials. More particularly, it relates to the fabrication of CuInxGa1-xS2 (0≦x≦1) nanoparticles.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
To be commercial viable, photovoltaic cells must generate electricity at a cost that is competitive with fossil fuels. The photovoltaic cells must be made using low cost materials and by inexpensive device fabrication processes. Photovoltaic cells must be capable of moderate to high conversion efficiency of sunlight to electricity. And the materials synthesis and device fabrication must be commercially scalable.
The photovoltaic market is currently dominated by silicon wafer-based solar cells (a.k.a. first-generation solar cells). The active layer in these solar cells is made of single-crystal silicon wafers having a thickness typically ranging from a few microns to hundreds of microns, a thickness that is relatively large. A thick active layer is required because silicon is relatively poor at absorbing light. These single-crystal wafers are relatively expensive to produce because the process involves fabricating and slicing high-purity, single-crystal silicon ingots. The yield of that process is often low.
The high cost of crystalline silicon wafers has prompted the industry to seek less expensive materials for solar cells. Semiconductor materials such as copper indium/gallium disulfides/selenides CuInxGa1-xS2 (0≦x≦1) (referred to herein generically as CIGS) are strong light absorbers and have bandgaps that match well with the optimal spectral range for photovoltaic applications. Furthermore, because these materials have strong absorption coefficients, the active layer in solar cells using these materials need be only a few microns thick.
Copper indium diselenide (CuInSe2) is one of the most promising candidates for thin film photovoltaic applications. However, solar cells based on CuInSe2 can be made by selenizing films of CuInS2. During selenization, films of CuInS2 are heated in a selenium-rich atmosphere, causing selenium to replace sulphur in some or all locations within the film, because when Se replaces S the substitution creates volume expansion, which reduces void space within the film and reproducibly forms a high quality, dense CuInSe2 absorber layer. Assuming complete replacement of S with Se, the resulting lattice volume expansion is approximately 14.6% (calculated based on the lattice parameters of chalcopyrite (tetragonal) CuInS2 (a=5.52 Å, c=11.12 Å) and CuInSe2 (a=5.78 Å, c=11.62 Å)). A CuInS2 nanoparticle film may be converted to a predominantly selenide material by annealing the film in a selenium-rich atmosphere. CuInS2 nanoparticles are a promising as precursors for producing the CuInSe2 active layer. An advantage of using CuInS2 nanoparticles instead of simply using CuInSe2 nanoparticles is that the sulphur precursors are usually less expensive and more readily available than their selenium counterparts.
The theoretical optimum bandgap for absorber materials is about 1.3-1.4 eV. By incorporating gallium into CuInS2 nanoparticles, the bandgap can be manipulated so that, following selenisation, the CuInxGa1-xSe2 absorber layer has an optimal bandgap for solar absorption.
Conventionally, costly vapor phase or evaporation techniques (e.g., organometallic chemical vapor deposition, RF sputtering, flash evaporation and the like) have been used to deposit the copper indium (gallium) disulfide films on substrates. While those techniques can produce high quality films, they are difficult and expensive to scale to large-area depositions and higher process throughputs.
One of the major advantages of using nanoparticles of copper indium chalcogenide and/or copper indium gallium chalcogenide is that the nanoparticles may be dispersed in a medium so as to form ink that may be printed on a substrate similar to inks in a newspaper-type process. The nanoparticle ink or paste may be deposited using low-cost printing techniques such as spin coating, slit coating or doctor blading. Printable solar-cells may replace the standard, conventional vacuum-deposited methods of manufacturing solar cells because the printing processes, especially when implemented in a roll-to-roll processing framework, enables a much larger throughput.
Nanoparticles of the ternary CuInS2 system have been prepared with various synthetic methods including the hot-injection method, solvothermal techniques, and thermal decomposition of suitable precursors. Colloidal nanoparticle synthesis typically employs high temperatures (above 250° C.), to form small (<20 nm), organic-capped nanoparticles. As such, colloidal nanoparticles display lower melting points than the bulk material. Such nanoparticles often have a narrow melting temperature range because the nanoparticles are highly monodisperse (i.e., the diameters of the nanoparticles are within a narrow size distribution). There is very little published literature on the synthesis of CuInGaS2 and CuGaS2 nanoparticles as the majority of the published literature focus on the ternary compound CuInS2.
U.S. Patent Application US 2011/0039104 A1 from Bayer (the '104 Application) describes a process for the colloidal synthesis of CuInS2 nanoparticles using copper salts, indium salts and an alkane thiol in a non-polar organic solvent at a reaction temperature between 240-270° C. is employed. The method described in the '104 Application does not demonstrate tunability to synthesise CuInxGa1-xS2 nanoparticles and does not demonstrate that tailoring of the initial metal ratio and choice of reagents can be used to obtain the desired stoichiometry. Further, to employ the reaction temperatures described in the '104 Application, a high-temperature boiling thiol is required.
In another example, Kino et al. report a method of synthesising CuInS2 nanoparticles by mixing Cu(OAc)2 and In(OAc)3 with 1-dodecanethiol and tri-n-octylamine at 230° C. [T. Kino et al., Mater. Trans., 2008, 49, 435]. Tri-n-octylamine is a highly coordinating solvent (with a boiling point of 365-367° C.), therefore it is likely that the nanoparticles synthesised using the Kito et al. method are at least partially amine-capped. If using the particles for photovoltaic devices, this is unfavourable since high processing temperatures are required to remove the amine from films made of the nanoparticles.
The hot-injection route usually consists of injecting a solution of sulphur in an appropriate solvent, such as trioctylphosphine (TOP) or oleylamine (OLA), into a solution of copper and indium salts at high temperature. Zn-doped CuInS2 nanoparticles have been prepared via this method at temperatures between 160-280° C. [H. Nakamura et al., Chem. Mater., 2006, 18, 3330]. A drawback of hot-injection techniques is that it is difficult to control the reaction temperature on a large scale, so reactions are generally restricted to milligram scales and typically require large reaction volumes.
Single-source precursor (SSP) methods for nanoparticle synthesis employ a single compound that contains all of the constituent elements to be incorporated into the nanoparticle. Under thermolysis, the SSP decomposes leading to nanoparticle formation. There are a number of references that describe the synthesis of CuInS2 nanoparticles from SSPs. CuInS2 nanoparticles were prepared by using precursors of the type (PR3)2Cu(SR)2In(SR)2, where R is an alkyl group. Castro et al. decomposed the liquid precursor (PPh3)2CuIn(SEt)4 in dioctyl phthalate between 200-300° C. to yield chalcopyrite CuInS2 nanoparticles of sizes between 3-30 nm [S. L. Castro et al., Chem. Mater., 2003, 15, 3142]. Despite their small size, the nanoparticles were insoluble in organic solvent due to their tendency of forming large 500 nm aggregates.
Dutta and Sharma used the xanthate precursors in(S2COEt)3 and Cu(S2COEt) in ethylene glycol at 196° C. to obtain tetragonal CuInS2 with an average size of 3-4 nm, with occasional aggregation [D. P. Dutta and G. Sharma, Mater. Lett., 2006, 60, 2395]. The CuInS2 nanoparticles prepared by these SSPs displayed very poor solubility and a tendency to form micron-sized aggregates because non-coordinating solvents were employed. SSP processes are complicated than other methods because they require an extra step to synthesise the precursors.
Other routes consist of reacting metal salts with a sulphur source. Choi et al. prepared Cu—In—S nanoparticles by decomposing copper and indium metal-oleate complexes with dodecanethiol in OLA at temperatures between 230-250° C. [S-H, Choi et al., J. Am. Chem. Soc., 2006, 128, 2520]. In this process the metal-oleate was synthesised, isolated and purified before being reacted with the alkyl thiol. The particles were fairly large with the particle shape being tailored to acorns, bottles, and larva-shape rods, with lengths between 50-100 nm, by changing the reaction time and temperature. However the XRD analysis revealed that the nanoparticles were composed of a mixture of hexagonal chalcocite-structured Cu2S and tetragonal-structured In2S3, rather than CuInS2. Carmalt et al. produced micron-sized CuInS2 particles by reaction of metal chlorides with sodium sulphide in refluxing toluene at 110° C., but this material had very limited solubility [C. J. Carmalt et al., J. Mater. Chem., 1998, 8, 2209].
Solvothermal methods have been explored as a route to nanoparticle synthesis. However, the particle size distribution is typically large and the nanoparticles are often poorly soluble due to the formation of aggregates. Micron-sized CuInS2 particles were prepared by mixing CuSO4, InCl3, and thioacetamide in the presence of thioglycolic acid inside an autoclave [X. Guo et al., J. Am. Chem. Soc., 2006, 128, 7222]. Lu et al. prepared tetragonal CuInS2 nanoparticles by reacting CuCl and metallic In with sulphur powder in a range of solvents including toluene, benzene, and water at 200° C. inside an autoclave [Q. Lu et al., Inorg. Chem., 2000, 39, 1606]. The particles had sizes between 5-15 nm, but formed large aggregates and were insoluble. Toluene, benzene or water was used as the reaction medium. TEM images showed poor control over the particle size distribution, which varied depending on the reaction medium. Additionally, Hu et al. report the solvothermal synthesis of CuGaS2 nanoparticles using CuCl, GaCl3 and thiourea [J. Q. Hu et al., Solid State commun, 2002, 121, 493].
A biomolecule-assisted synthesis of CuGaS2 nanoparticles was reported by Zhong et al. [J. Zhong et al., Appl. Surf. Sci., 2011, 257, 10188]. CuCl2.2H2O, GaCl3 and L-cysteine (C6H12N2O4S2) were dissolved in ethylenediamine and water then stirred at room temperature for 20 minutes. The solution was heated to 200° C. in an autoclave for 10 hours, TEM analysis showed large nanoparticles with an average diameter of 600 nm.
Though there is substantial interest in developing synthetic methods to provide quaternary and higher nanoparticles, few methods of CuInS2 and/or CuGaS2 nanoparticle synthesis in the prior art have been proven to be adaptable to provide CuInxGa1-xS nanoparticles across the 0≦x≦1 range. Wang et al. describe the colloidal synthesis of wurtzite CuInxGa1-xS2 nanoparticles in the range 0≦x≦1 [Y-H. A. Wang et al., J. Am. Chem. Soc., 2011, 133, 11072]. Cu(acac)2, In(acac)3, Ga(acac)3 and trioctylphosphine oxide (TOPO) were stirred in OLA at room temperature and purged with nitrogen for 30 minutes. The solution was heated to 150° C., then 1-dodecanethiol (DDT) and tert-DDT were injected rapidly into the solution, which was then heated to 280-290° C. in 30 minutes, then held for 30 minutes. The solution was cooled to room temperature then isolated by centrifugation using hexane and ethanol. By substituting OLA with 1-octadecene (ODE), the nanoparticle morphology could be changed from bullet-like to tadpole-like. Morphological variation was also observed when changing the In:Ga ratio. The authors claim that the wurtzite phase offers flexibility to control the stoichiometry of the material.
Though the method outlined by Wang et al. can be used to synthesise CuInxGa1-xS2 nanoparticles across the entire 0≦x≦1 range, the nanoparticles are capped with high boiling ligands: OLA (348-350° C.), TOPO (201-202° C. at 2 mm Hg, which equates to 397-399° C. at atmospheric pressure), 1-DDT (266-283° C.) and/or tert-DDT (227-248° C.). Thus, high temperature device manufacturing techniques are required to remove the ligand from the resulting films. Moreover, with the Wang et al. method, the uncommon wurtzite phase of CuInS2 is obtained. In contrast, current solar cells use the chalcopyrite phase as the absorber.
Chang et al. describe the synthesis of the quinary Cu(InxGa1-x)(SySe1-y)2 nanoparticles in the range 0≦x,y≦1, which enables tuning of the bandgap from 0.98-2.40 eV [S-H, Chang et al., Energy Environ. Sci., 2011, 4, 4929]. In a typical reaction, CuCl, InCl3 and/or GaCl3, Se and/or S were mixed with OLA, then purged with Ar at 130° C. for 1 hour under vigorous stirring. The solution was heated to 265° C. then held for 90 minutes, after which the reaction was quenched in a cold water bath. The product was isolated by centrifugation with hexane/ethanol. For x,y˜0.5, the average particle diameter was 16±0.5 nm, with a slightly irregular faceted morphology. A similar reaction is reported by Guo et al. Colloidal CuInxGa1-xS2 nanoparticles were synthesised with an average particle diameter of 15 nm when x=1 [Q. Guo et al., Nano Lett., 2009, 9, 3060]. In a typical synthesis, CuCl, InCl3 and/or GaCl3 were dissolved in OLA and purged under Ar at 130° C. for 30 minutes. The solution was heated to 225° C., then a 1 M S/OLA solution was injected in quickly. The reaction was held at 225° C. for 30 minutes, then cooled and isolated by centrifugation with toluene/ethanol. The resulting nanoparticles have a very low organic content (<10%), making them insoluble in organic and polar solvents and difficult to process as a printable ink.
Combinations of In-containing and Ga-containing SSPs have been used to synthesise Cu(In,Ga)S2 nanoparticles. Sun et al. used a mixture of two single source precursors, (Ph3P)2Cu(μ-SEt)2In(SEt)2 and (Ph3P)2Cu(μ-SEt)2Ga(SEt)2, in varying ratios to synthesise CuInxGa1-xS2 nanoparticles across the 0≦x≦1 range [C. Sun et al., Chem. Mater., 2010, 22, 2699]. In a typical synthesis, (Ph3P)2Cu(μ-SEt)2In(SEt)2 and (Ph3P)2Cu(μ-SEt)2Ga(SEt)2 were dissolved in benzyl acetate in the presence of 1,2-ethanedithiol, then irradiated in a microwave at 160° C. for less than 1 hour. Microwave irradiation was employed to provide greater homogeneity in the reaction temperature than traditional thermolysis. Nanoparticle diameters ranged from 2.7-3.3 nm, increasing with an increase in In content, and the bandgap could be tuned from 1.59 eV (for x=1) to 2.3 eV (for x=0). Increasing the reaction temperature was shown to increase the particle size and decrease the band gap. Though the SSP method described by Sun et al. allows tunability of the In to Ga ratio, the ratio of Cu to (In+Ga) is determined by the stoichiometry of the single source precursors and cannot be altered.
A U.S. Pat. No. 7,892,519 describes an SSP method for producing Cu(In,Ga)S2 nanoparticles capped with a thiolate ligand. However the disclosure only exemplifies methods to synthesise CuInS2.
The solvothermal synthesis of CuIn0.5Ga0.5S2 1-2 μm flowers, consisting of nanoflakes of 15 nm thickness, is described by Liang et al. [X. Liang et al., J. Alloys & Compounds, 2011, 509, 6200]. In a typical reaction, CuCl2.2H2O, GaCl3, InCl3, and L-cysteine were dissolved in DMF by stirring for 10 minutes. The solution was heated in an autoclave at 220° C. for 10 hours, then cooled to room temperature. The solid was precipitated with deionised water, then dried under vacuum.
The synthetic methods described in the prior art generally produce large nanoparticles that have a tendency to aggregate and are insoluble in most solvents. This is an important issue because it is desirable to produce small and soluble nanoparticles that may be further processed to formulate an ink to make inorganic films by conventional and low-cost techniques like printing or spraying. Capping ligands, such as hydrocarbons, can be associated with the surface of the nanoparticles to aid in the processability. However, the synthetic procedures described above are carried out at a high temperature, which limits the choices of capping ligands to those ligands having a relatively high vaporization/decomposition temperature. The presence of such low volatility capping ligands complicates the use of the nanoparticles for the preparation of photovoltaic films because it is difficult to remove the ligands during the sintering of the films. The presence of removed ligand in the film results in carbon-based impurities, which adversely affect the performance of the films.
Because the known methods are not able to produce nanoparticles that are small, have low melting points, narrow size distribution, and incorporate a volatile ligand that may confer solubility and processability, these methods are not very well-suited for producing nanoparticles compatible with the conventional low-cost film printing techniques. Further, few methods have been proven successful for synthesising CuInxGa1-xS2 nanoparticles spanning the entire 0≦x≦1 range. An object of the present disclosure is to solve these problems.